Obesity occurs when caloric intake exceeds expenditure. The increasing prevalence of obesity in industrialized societies has generated new interest in the mechanisms underlying the control of energy balance. In humans, reduced rate of energy expenditure, which is largely accounted for by oxidative phosphorylation in skeletal muscle, is a risk factor for future weight gain. Nutrients, via the activation of nutrient-sensing pathways, activate adipostatic responses (e.g. leptin) that attempt to limit lipid storage by enhancing energy expenditure and inhibiting food intake. A major nutrient-sensing pathway, the hexosamine biosynthetic pathway (HBP) has been recently shown to increase local leptin expression in skeletal muscle. Yet, acute pharmacological activation of HBP induces a dramatic drop in skeletal muscle energy production, by downregulating the expression of genes for oxidative phosphorylation. Based on these observations, we wish to develop animal models designed to define how the physiological regulation of HBP modulates energy homeostasis. In particular, we will use transgenic animal models to discern the metabolic consequences of activation of HBP in muscle from those derived from the associated local induction of leptin. We will generate transgenic mice lines carrying constitutive and moderate elevations of the enzyme glutamine:fructose 6-P amidotransferase (GFAT, which catalyzes the first committed step of HBP and regulates the flux through this pathway), or of leptin in skeletal muscle. The careful phenotyping of these animal models should allow one to define the direct and respective roles of muscle HBP and leptin in the regulation of energy expenditure and substrate oxidation.